1. Field of the Invention
The present invention relates to so-called water-proof type oxygen sensors each comprising a sensor element having inner and outer electrodes at inner and outer surfaces thereof, respectively, which inner and outer electrodes are to contact with surrounding air and exhaust gases, respectively; a metallic cap for housing the sensor element; a gas-tight sealing member arranged inside the cap for isolating the exhaust gases from the surrounding air, and a communicating member having a water-repelling property for communicating the surrounding air with the air inside the gas-tight sealing member.
2. Related Art Statement
Water-proof type oxygen sensors conventionally have been known as oxygen concentration detectors for exhaust gases from automobiles.
Since these oxygen sensors use surrounding air as a reference oxygen atmosphere, they are constructed to isolate the air as the reference oxygen atmosphere from the gases to be measured. In order to isolate air from the gases to be measured, in the case; of an oxygen sensor 10 as shown in FIG. 4, a gas-tight sealing member, for example, talc 4 is charged between a planar sensor element 2 placed in a metallic housing 1 and a metallic cap 3.
In this manner, an inner electrode is isolated from the gases to be measured, and an air introduction path is provided for introducing the surrounding air into the reference oxygen electrode.
As shown in FIG. 4, the air introduction path is constituted by communicating openings 5 provided in the metallic cap 3, and a column-like communicating member 6 arranged inside the metallic cap 3 and having gas permeability and water-repelling property, while the metallic cap 3 is caulked from the externally peripheral side to intimately adhere the inner surface of the metallic cap 3 with the peripheral surface of the communicating member 6. By this construction, air can be fed to the reference oxygen electrode, while the water-proof structure is maintained.
However, when the water-proof type oxygen sensor having such a construction is repeatedly heated at high temperatures and cooled during use for a long time period, a gap is formed between the metallic cap 3 and the communicating member 6 (generally made of a resinous material) due to difference in thermal expansion between them.